Such a tool may be used to work at a mine face on a seam of anthracite coal, for example, whose fragmentation by the rotating cutters gives rise to considerable dust development. Continuous sprinkling of the detritus with water enables the dust to be precipitated and to be carried off for further processing. For economic reasons, and to simplify the recovery by avoiding the need for a separate drying step, it is desirable to hold the water supply within moderate limits, e.g. at or below 6% by weight of the extracted coal. Thus, rotary heads carrying axially projecting picks have been provided in the past with valves which block the flow as long as the picks are not in contact with a mine face. The valve responds to axial pressure and thus unblocks the flow intermittently as the tool is progressively advanced into the seam.
When the cutters project generally radially from the periphery of the rotary body, e.g. with shearer-type mining tools or augers as described for example in U.S. Pat. Nos. 3,747,982 and 4,219,239, such a control cannot be used since the cutters are not axially shiftable. Moreover, each individual cutter comes into working contact with the mine face only during part of a revolution. The cutters, therefore, need not be supplied with water during their nonworking phases. For this purpose it is known, e.g. as noted in the aforementioned U.S. Pat. No. 3,747,982, to control the water supply by means of a distributor cutting off the flow to a segment remote from the mine face, yet this will not prevent the unneeded emission of water from an opposite segment when the cutters thereof are not in contact with the mine face.